docs: def _col_name(index):
      """
      Converts a column index to a column name.
  
      >>> _col_name(0)
      'A'
      >>> _col_name(26)
      'AA'
  
      """
      for exp in itertools.count(1):
          limit = 26 ** exp
          if index < limit:
              return ''.join(chr(ord('A') + index // (26 ** i) % 26) for i in range(exp-1, -1, -1))
          index -= limit